Paste Extrusion Research Articles

Preparation and properties of porous polytetrafluoroethylene hollow fiber membrane through mechanical operations

ABSTRACTPorous polytetrafluoroethylene (PTFE) hollow fiber membranes were prepared from fine powder through a series of mechanical operations including paste extrusion, heat treatment, stretching and sintering. In contrast to conventional process, the heat treatment used in this study was performed at 200°C to 330°C (near the melting point) on the PTFE nascent hollow fiber (precursor of membrane). The results showed that the introduction of heat treatment step effectively improved the mechanical properties of precursors, the ultimate stress and strain increased observably with heat treatment temperature, which was beneficial to subsequently stretching precursors to make them porous. Furthermore, the morphological changes and improvement of membrane properties caused by stretching operation were investigated for porous PTFE hollow fiber membrane having finer pore size and higher porosity. The porous microstructure of nodes interconnected by fibrils varied depending on the stretching conditions, such as the stretching temperature, rate, and ratio. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015, 132, 42696.

Manufacturing of metal-organic framework monoliths and their application in CO2 adsorption

An important class of novel mesoporous and microporous adsorbents like metal-organic frameworks (MOFs) are normally produced in powder form. This paper presents a generic method of manufacturing and characterisation of these materials into low pressure drop and energy saving monolithic structures for industrial applications. One of the MOF candidates that was considered in this study was MIL-101 (Cr) ([Cr3O(OH)(H2O)2(bdc)3].xH2O; bdc = 1,4-benzenedicarboxylate), and the model contaminant gas tested was carbon dioxide (CO2). MIL-101 (Cr) monoliths were manufactured by paste extrusion techniques from the synthesized MIL-101 (Cr) powder. These MIL-101 (Cr) monoliths were then characterised using powder X-ray diffraction (PXRD), scanning electron microscopy (SEM), mercury intrusion porosimetry (MIP), radial compression tests and intelligent gravimetric analysis (IGA). Adsorption properties of the prepared MIL-101 (Cr) powder and monoliths were determined from their pure CO2 sorption isotherms and dynamic adsorption breakthrough curves, that were carried out using high concentration (40% v/v) CO2 challenge. Results have demonstrated that the resulting MIL-101 (Cr) monoliths were highly porous, mechanically strong on compressive loading, thermally regenerable with comparable CO2 adsorption capacity to the synthesized MIL-101 (Cr) powder. From breakthrough curves, mass transfer characteristics such as mass transfer zone velocity and length of the prepared MIL-101 (Cr) monoliths have also been evaluated in this study.

Mixing quality of powder-liquid mixtures studied by near infrared spectroscopy and colorimetry

In many industries, mixing powder with a low amount of liquid is a frequently used process step, e.g. in order to generate agglomerate/granule product structures or for sintering and extrusion of wet powders or pastes. There are still great difficulties in achieving a homogeneous mixture due to local agglomeration and lump formation. A new mixing methodology has therefore been developed, in which the liquid is transformed into a solid powder by spray chilling, generating so-called powder-liquids. These powder-liquids are then mixed with other dry powders.An even liquid distribution, as well as the related even distribution of functional components in the final product is of high relevance. We thus aimed to evaluate the mixing quality of such powder/powder-liquid mixtures studied by near infrared (NIR) spectroscopy and colorimetry. For this purpose we quantified the homogeneity of fat based powder-liquids mixed with flour. NIRS is a well-recognized quantification method that successfully has been used to analyze powder mixtures before, while colorimetry is a method less used for such an application. The results demonstrated that both methods are applicable for quantifying the mixing behavior of such powder mixtures. We showed that cheap, easy and fast colorimetry is as reliable as NIRS measurements and hence can be a first choice validation method in industry. Comparing the powder-liquid mixing with spraying the liquid directly into the powders, the powder-liquids mixed faster and resulted in better homogeneity. Our results support the use of powder-liquids to process new highly homogeneous products required in functional foods or pharmaceutical products.

Honeycomb Structures of TiO2-modified Hydroxyapatite Composite for Microbial Filtration Application

Honeycomb structures of TiO2-modified hydroxyapatite composite (HA/TiO2) were fabricated using extrusion technique. The mixture formulations of HA/TiO2 extrusion pastes (S1-S4) were investigated to optimize the process. The filtering effectiveness of the HA/TiO2 honeycombs was evaluated by photocatalytic activity measurement and a bactericidal test. The effects of sintering temperature and honeycomb thickness on photocatalytic and antibacterial activities were additionally studied. The S3 and S4 honeycomb samples, sintered at 650°C, exhibited a similar trend in reducing methylene blue concentration. At 800°C, the S3 honeycomb sample showed a slightly faster reduction in methylene blue concentration compared to the S4 honeycomb. Honeycomb samples of 2 and 5 cm in thickness showed significantly greater photocatalytic activity than that of 1 cm such that methylene blue concentrations rapidly decreased after UV exposure for 24 hours. The S3 and S4 honeycomb samples also exhibited decomposition of both gram-negative E. coli and gram-positive S. aureus. Approximately 50% of gram-positive S. aureus and gramnegative E. coli were decomposed by the honeycombs in the sample-thickness dependent manner after 2 hours of UV exposure. Low survival ratios of bacteria (5% - 10%) were observed after 6 hours of UV exposure and the 2 mm and 5 mm thick honeycomb samples showed the greatest and most similar antibacterial activities.

Extrusion of Fe<sub>2</sub>O<sub>3</sub>/SBA-15 mesoporous material for application as heterogeneous Fenton-like catalyst

The aim of this work has been the extrusion of powder Fe₂O₃/SBA-15 catalyst in order to be successfully used in continuous catalytic fixed bed reactors as Fenton-like catalyst. The extrusion method was optimised using an amorphous silica material of similar properties than the Fe₂O₃/SBA-15 catalyst. The main studied variable was the composition of the extrusion paste using bentonite and methylcellulose as inorganic and organic binders, respectively. The organic content displayed a significant influence on the mechanical strength and specific surface area of the final extrudates. In contrast, the inorganic binder content hardly affected the final properties (in the studied range). The extruded Fe₂O₃/SBA-15 material showed a remarkable mechanical strength as well as the typical mesoporous structure of Fe₂O₃/SBA-15 with a relevant specific surface area (264 m²/g). The extruded catalyst achieved a high catalytic performance in the catalytic wet peroxide oxidation of phenol with a 60 % of total organic carbon reduction in both batch and continuous processes.

Development of Freeze-Form Extrusion Fabrication With Use of Sacrificial Material

The development of freeze-form extrusion fabrication (FEF) process to fabricate three-dimensional (3D) ceramic parts with use of sacrificial material to build support sections during the fabrication process is presented in this paper. FEF is an environmentally friendly, additive manufacturing (AM) process that builds 3D parts in a freezing environment layer-by-layer by computer controlled extrusion and deposition of aqueous colloidal pastes based on computer-aided design (CAD) models. Methyl cellulose was identified as the support material, and alumina was used as the main material in this study. After characterizing the dynamics of extruding alumina and methyl cellulose pastes, a general tracking controller (GTC) was developed and applied to control the extrusion force in depositing both alumina and methyl cellulose pastes. The controller was able to reduce the time constant of the closed-loop system by more than 65% in comparison to the open-loop control system. Freeze-drying was used to remove the water content after the part has been built. The support material was then removed in the binder burnout process. Finally, sintering was done to densify the ceramic part. The fabrication of a cube-shaped part with a square hole in each side that requires depositing the sacrificial material during the FEF process was demonstrated.

Stages in spheronisation: Evolution of pellet size and shape during spheronisation of microcrystalline cellulose-based paste extrudates

The stages of extrudate breakup and rounding to form spheroidal pellets were investigated for MCC/water pastes and some MCC/water/calcium carbonate pastes using the interrupted technique reported by Lau et al. (2014). A new quantitative parameter, named ‘dumb-bellity’, was developed to monitor the formation and disappearance of ‘dumb-bell’ shaped pellets in the early stages of the rounding process. Tests using mixtures of coloured extrudates confirmed that attachment of small fragments (‘fines’) to the waist of pellets was not responsible for the transition from dumb-bell to more spheroidal shapes. The results confirmed the findings of Lau et al., that rounding was the rate-limiting step. Extrudates prepared with up to 20wt.% calcium carbonate, (the carbonate representing a hard, active pharmaceutical ingredient), were subject to the same spheronisation mechanism. The time to spheronise the carbonate-containing pastes was longer, which could be related to the increased deformation resistance of these materials.

Antioxidant potential and quality characteristics of vegetable-enriched corn-based extruded snacks.

Phenolic content, antioxidant activity and sensory characteristics of vegetable-enriched extrudates were investigated as a result of extrusion conditions, including extrusion temperature (140-180°C), screw rotation speed (150-250rpm) and feed moisture content (14-19% w.b.). Broccoli flour and olive paste was used in mixtures with corn flour at a ratio of 4 to 10% (broccoli/corn) and 4 to 8% (olive paste/corn). A simple power model was developed for the prediction of phenolic content and antioxidant activity of extrudates by extrusion conditions and feed composition. Phenolic content and antioxidant activity of broccoli enriched extrudates increased with extrusion temperature and broccoli addition and decreased with feed moisture content. The antioxidant activity of olive paste extrudates increased with material ratio and decreased with feed moisture content and screw rotation. Sensory porosity, homogenous structure, crispness, cohesiveness and melting decreased with feed moisture content, while the latter increased the mealy flavor and hardness of extrudates. Acceptable snacks containing broccoli flour or olive paste can be produced by selecting the appropriate process conditions.

Effect of agglomerate size on extrudability of titania pastes

Pastes prepared from three high surface area titania powders were tested via torque and capillary rheometry to explain the different flow behaviour during mixing and extrusion processing. The Benbow analysis was used to calculate the main extrusion parameters, including bulk yield and shear stresses, and to compare the processability of the extrudable pastes. A critical water fraction was found for all powders and related to extrudability. The results show that the flow properties of the pastes are strongly influenced by the particle size distribution and the shear stability of large aggregates of different dimensions in the paste microstructure.

The shear dependence of the methylcellulose gelation phenomena in aqueous solution and in ceramic paste

The gelation temperature of methylcellulose (MC) in aqueous solutions as well as in aqueous ceramic paste depends on the applied shear. Rheological investigations in oscillation vs. shear mode show lower gelation temperature at low shear rates as for the corresponding angular frequencies. Above a critical shear rate the gelation temperature is shifted to higher temperatures. The paste extrusion process uses MC as a plasticizer and runs under high shear conditions. When extruding close to the gelation temperature of the MC in the paste, crack formation and other defects can occur. The upwards shift of the gelation temperature with increasing applied shear gives a larger temperature window during the extrusion process. The understanding of the shear influence on the gelation temperature is important to design the optimal process conditions.

Novel method for the identification of the maximum solid loading suitable for optimal extrusion of ceramic pastes

Gadolinia-doped ceria ceramic pastes were formulated with different solid loadings and extruded using lab-scale equipment. The force to maintain a constant ram speed of 10 mm/min was recorded. The radial shrinkage after drying was proportional to the solid loading and this allowed the determination of the maximum solid loading by an extrapolation procedure. In order to obtain the apparent viscosity of the pastes, a novel approach based on the analysis of the slope of the extrusion pressure plot versus distance covered by the ram, was formulated for the direct determination of the shear stress upon extrusion. The agreement of the determined maximum solid loading with values calculated by two existing models confirmed that the proposed approach was an alternative and reliable method to identify the upper limit of the solid loading range for the formulation of extrudable ceramic pastes.

Effect of stretching ratio and heating temperature on structure and performance of PTFE hollow fiber membrane in VMD for RO brine

Membrane distillation (MD) using hollow fiber membrane for desalination is becoming an attractive technology due to high water recovery and salt rejection. Polytetrafluoroethylene (PTFE) hydrophobic hollow fiber membranes were prepared through a cold pressing method including paste extrusion, stretching and heating. The influences of stretching ratio and heating temperature on the morphology, structure, porosity, shrinkage ratio, tensile strength and permeability of the PTFE hollow fiber membranes were investigated. Field emission scanning electron microscopy (FESEM) images showed that the PTFE hollow fiber membranes had the microstructure of nodes interconnected by fibrils. The increase of stretching ratio can improve the pore size and porosity of the membrane. The heating temperature higher than the melting point of PTFE was necessary in the heating process because melting treatment can restrain the shrinkage and improve the tensile strength of the membrane. Finally, the PTFE hollow fiber membranes were tested for their performances in desalination through vacuum membrane distillation (VMD) for the treatment of seawater reverse osmosis (RO) brines. The increase in stretching ratio and heating temperature significantly improved the permeation flux. The salt rejections achieved 99.9%, indicating the prepared PTFE hollow fiber membranes exhibited satisfying salt rejection performances. The hydrophobic PTFE hollow fiber membranes developed in this study are promising for practical application in the treatment of seawater RO brine.

Prediction of extrusion load and liquid phase filtration during ram extrusion of high solid volume fraction pastes

For perfect and homogeneous plastic materials, the extrusion stress and flow typology have previously been well described. However for firm pastes, during extrusion, liquid phase migration may appear leading to unsuccessful processing. Reviewed literature explains that there is a competition between the extrusion velocity and the liquid filtration velocity (or time).The aim of this work is to develop a new model to predict the ram extrusion force of frictional plastic materials such as firm fresh cement-based materials or clay pastes. A quantitative one-dimensional model based on consolidation theory has been developed to predict the extrusion force and quantify liquid phase migration.Model results are compared with experimental data from tests carried out on kaolin paste undergoing ram extrusion. Influences of extrusion rate, rheological parameters and filtration characteristics on extrusion load are investigated.

Hybrid Extrusion Force-Velocity Control Using Freeze-Form Extrusion Fabrication for Functionally Graded Material Parts

Freeze-form extrusion fabrication (FEF) is an additive manufacturing (AM) process that uses an aqueous-based paste loaded with ceramic or metal powder to build complex, three-dimensional parts by extruding the material from a syringe onto a solid substrate in a subzero temperature environment. This paper describes the development of an intelligent control methodology for paste extrusion that utilizes a hybrid extrusion force-velocity controller. A plunger velocity controller was used to ensure a steady extrusion flow rate, and an extrusion force controller was developed to precisely regulate the starting and stopping of the extrusion process. Both controllers were coupled with a hybrid control scheme for extrusion-on-demand and air bubble release compensation. The plunger velocity controller successfully regulated the output material composition from two syringes, and the extrusion force controller precisely controlled the extrusion start and stop. Air bubble release compensation reduced the severity of extrusion gap defects and extrusion track thinning resulting from air bubble releases. Monolithic and functionally graded parts were fabricated to illustrate the functionality of the hybrid extrusion force-velocity controller.

Polytetrafluoroethylene Paste Extrusion: A Fibrillation Model and Its Relation to Mechanical Properties

The effects of process conditions on fibrillation and mechanical properties of polytetrafluoroethylene (PTFE) paste extrudates have been studied using capillary rheometers having barrels of different diameter and equipped with capillary dies of various designs. The tensile strength of PTFE extrudates is measured as a function of apparent shear rate (flow rate), reduction ratio (cross sectional area of barrel to that of die), contraction angle, and diameter of the barrel. To describe the effects of die design on the quality of the final product, a basic phenomenological mathematical model has been developed. The model consists of a simple equation that explains fibril formation, due to the compression of PTFE resins, plus a kinetic equation, which is coupled with the “radial-flow” hypothesis to predict the structure and the tensile strength of extrudates. The model predictions are found to be consistent with tensile strength measurements and SEM micrographs of the PTFE extrudates.

Preparation and properties of PTFE hollow fiber membranes for desalination through vacuum membrane distillation

Polytetrafluoroethylene (PTFE) hydrophobic hollow fiber membranes were prepared through a cold pressing method including paste extrusion, stretching and sintering. The effect of stretching ratio on the characteristics of PTFE hollow fiber membranes was investigated. The morphological property of the PTFE hollow fiber membrane was studied in terms of field emission scanning electron microscopy (FESEM). It was observed that the PTFE hollow fiber membranes with four stretching ratios (120%, 160%, 180% and 220%) showed the microstructures of nodes interconnected by fibrils. Increasing stretching ratio was positive for pore size and porosity but negative for water entry pressure and mechanical property. The result of water contact angle measurement showed that the prepared PTFE hollow fiber membrane was a superior hydrophobic membrane material. Finally, the PTFE hollow fiber membranes were tested for their performances in the process of desalination through the vacuum membrane distillation (VMD). An increase in stretching ratio significantly improved the permeation flux. The salt rejections achieved 99.9% for all the PTFE hollow fiber membranes with four stretching ratios in this study, indicating the prepared PTFE hollow fiber membranes exhibited satisfying salt rejection performances.

Pasting and Extrusion Properties of Mixed Carbohydrate and Whey Protein Isolate Matrices

Mixed systems of whey protein isolate (WPI) or texturized WPI and different starches may form weak or strong gel pastes or rigid matrices depending on interactions. The paste viscoelasticity of starches from amioca, barley, cornstarch, Hylon VII, plantain and pea starch, mixed with whey protein isolate or textured WPI at the concentrations of 75/25, 50/50 or 25/75 wt% was determined using a Rapid Visco Analyzer. Peak paste ranged from plantain (5.33 Pa·s), amioca (4.14 Pa·s), cornstarch (3.65 Pa·s), barley (2.61 Pa·s), Hylon VII and pea starch (10 mPa·s). The presence of whey proteins reduced paste viscosity and extrusion expansion, but reduced hardness and improved fracturability. Moisture content was a stronger determinant of paste properties and of the denatured state of the whey protein isolate. Practical Applications Foods may contain starches from different carbohydrate and protein sources. Food formulators sometimes combine different proteins and carbohydrate for desired textural benefits. Our study shows that some protein–starch interaction synergies depend on the source of starch and the amount of protein isolate admixed. This knowledge is useful to manufacturers who may seek to enhance food texture by blending different proteins.

Modeling Powder Extrusion Pastes for Forming 410L Stainless Metallic Honeycombs

A thin-wall 410L Stainless metallic honeycomb has been fabricated successfully by extruding and sintering processing. The extruded pastes are a combination of two phases: a solid phase composed of metallic powder carried by a fluid solution of water, binder and additives. The key to forming high quality, defect free honeycombs lies in the optimization of paste properties and is contingent on solids loading and fluid-phase rheology. To develop a model that predicts paste rheology, capillary rheometry was used to characterize powder pastes and binder gels used as the fluid phase. Suspension viscosity models were successfully applied to permit rapid optimization of solids content and binder gel solution for extrusion of honeycombs.

Rheology of semi-solid fresh cement pastes and mortars in orifice extrusion

Short fiber-reinforced semi-solid fresh cement pastes and mortars, tailored for extrusion, have much lower water-to-binder ratio and higher viscosity than normal cement pastes or mortars. The rheology of these pastes or mortars cannot be characterized by traditional rheology test methods suitable for normal fresh cement pastes or mortars with much greater water-to-binder ratio and lower viscosity. In this paper, orifice extrusion is employed to calibrate rheology of the semi-solid fresh cement mortar. An analytical model is developed for orifice extrusion of semi-solid pastes and mortars obeying a rigid-viscoplastic constitutive relationship, von-Mises yield criterion and the associated flow rule. Orifice extrusion results are interpreted using the analytical model and the established experiment data interpretation method, and the associated rheological parameters are derived for the semi-solid fresh cement mortar. This study provides a simple analytical model, together with experiment and data interpretation methods, for characterizing the complex intrinsic rheological behavior of semi-solid fresh cement pastes or mortars.

Factors influencing paste extrusion pressure and liquid content of extrudate in freeze-form extrusion fabrication

Freeze-form extrusion fabrication process extrudes an aqueous ceramic paste of high solids loading by ram extruder to fabricate 3-D ceramic green parts. An appropriate extrusion pressure is necessary to get the desired extrusion velocity, and the extrudate should be keep in the same composition during the extrusion process for fabricating uniform green parts. The ram velocity, die land geometry, and paste property are three main factors influencing paste extrusion process. In this paper, the experiments were carried out to determine the effect of the three factors on the extrusion pressure and liquid content of extrudate. The experimental results show that the ram velocity is a strong factor governing the extrusion pressure profiles and too low ram velocity can result in liquid phase migration. The die land diameter and paste viscosity have obvious impact on paste extrusion process, which is largely consistent to the Benbow–Bridgwater model but sometimes is influenced by liquid phase migration. A further analysis of paste extrusion process indicates that the varying of paste structure in different extrusion stage leads to the experimental phenomenon. The research can help to get better understanding of influence factors in paste extrusion and to give reference of controlling the process in freeze-form extrusion fabrication process.